Oct. 1, 2019:
DM Radio Cubic Meter is a full-scale QCD axion search that will probe QCD axion dark matter over 1.5 orders of magnitude of mass. It is a Consortium of SLAC, Stanford, UC Berkeley, LBNL, MIT, UNC Chapel Hill, and Princeton university.
In order to detect ever smaller signals, detector arrays of all types try to add more and more pixels. For cryogenic transition edge sensor (TES) detectors, adding additional pixels comes with a few complications. More pixels means more wires, which add complexity and conduct heat from room temperature to the cryogenic stage. Detectors with hundreds of pixels are large enough that it is already impractical to run individual wires to each pixel, and the problem only becomes harder for kilopixel or megapixel arrays. In order to fabricate larger and larger arrays, it is necessary to read multiple pixels with a single pair of signal wires. The Irwin group has been involved in the development of many current-generation readout schemes, and are actively developing the next generation of technology to drive towards kilopixel and megapixel readout.
The Irwin group makes use of Superconducting Quantum Interference Devices (SQUIDs) to multiplex many TES signals onto one cable to room temperature. In microwave SQUID multiplexing, each TES is inductively coupled to a SQUID, which acts as flux-variable inductor. When this inductance is part of a high quality-factor resonance, the flux in the SQUID shifts the resonance frequency, allowing the current in the TES to be inferred. Each resonator is tuned to a different resonance frequency, allowing efficient use of the bandwidth available to cryogenic High Electron Mobility Transistor (HEMT) amplifiers. The schematic below gives an outline of the overall measurment scheme: a comb of interrogation tones (blue and green frequencies) enter the cryostat, and interacts with the resonators (color coded). The resonators imprint the TES current signal (coupled from the inductances at the bottom of the image) onto the phase and amplitude of each tone (rotated blue and green frequencies), which can be processed at room temperature.
(Image credit Ben Mates)